Polarography

instrumentation, application and principle of polarography

1. POLAROGRAPHY Madhuri Shelar (M pharm ) Assistant Professor Alard College of Pharmacy

2. POLAROGRAPHY - an electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solution as well as the gradually increasing applied voltage to determine respectively the concentration of a solute and its nature. -created by: Jaroslav Heyrovsky

3. POLAROGRAPHY  “Polarographic Analysis”  Is a method of analysis based on the measurement of current electrolysis of an electroactive species at a given electrode potential under controlled conditions.  It is the branch of voltammetry where the working electrode is a dropping mercury electrode (DME) or a static mercury drop electrode (SMDE), which are useful for their wide cathodic ranges and renewable surfaces.

4.  In this method, a reference electrode and an indicator electrode are required.  Reference electrode- acts to maintain a constant potential throughout the measurement.  Indicator electrode- assumes the potential impressed upon it from an external source. Reference electrode Indicator electrode

5. EXAMPLES OF MERCURY ELECTRODES  3 1. In polarography, mercury is used as a working electrode, because mercury it is a liquid. The working electrode is often a drop suspended from the end of a capillary tube. examples of electrodes: HMDE (Hanging mercury drop electrode)- we extrude the drop of Hg by rotating a micrometer screw that pushes the mercury from a reservoir through a narrow capillary. DME ( dropping mercury electrode) - mercury drops form at the end of the capillary tube as a result of gravity. Unlike the HMDE, the mercury drop of a DME grows continuously— as mercury flows from the reservoir under the influence of gravity—and has a finite lifetime of several seconds. At the end of its lifetime the mercury drop is dislodged, either manually or on its own, and replaced by a new drop. DSME ( static mercury drop electrode)- uses a solenoid driven plunger to control the flow of mercury. Activation of the solenoid momentarily lifts the plunger, allowing mercury to flow through the capillary and forming a single, hanging Hg drop.

6. theory The theory involved in polarography is when the working electrode is dipped in the analyte solution containing electro-active species, the following reduction takes place: A (OX) + ne − A (RED) Example: Cu +2 + 2e − Cu The reduced potential is created on the working electrode. The movement of the ions from the solution to the electrode is by three mechanisms. They are as follows: Convection: This is also known as discharge process. This is carried out by the stirring of the sample solution at a constant temperature. Migration: Here movement of particles due to attraction of force of the electric field is created by the electrode. Diffusion: Here spontaneous movement of the sample ions occurs based on the concentration gradient. The movement of the sample ions is controlled by the placement of the supporting electrolyte solution.

7. This supporting electrolyte solution surrounds the electrode with ions. The supporting electrolyte should posses the following ideal requirements: It should be chemically inert. It should have different discharge potentials. It should have ionic conductivity. The total current flowing is given by the following equation: I = I d + I m where I is the total current; I d is the diffusion current; I m is the migration current. The diffusion rate of the ion on the electrode surface is stated by Fick's second law: δ c / δ t = D δ 2 c /δ x 2 where D is the diffusion coefficient; C is the concentration; t is the time; x is the distance from the electrode surface.

8. 8 Current is a function of analyte concentration how fast analyte moves to electrode surface rate of electron transfer to sample voltage, time... Readout voltage Detector/ Transducer/ Sensor signal Excitation Process Sample Voltage is applied to analyte; appreciable current is measured View current as a function of time or applied voltage Current is transformed to voltage by electronics Concept

9. PRIN C IPLE  Study of solutions or of electrode processes by means of electrolysis with two electrodes, one polarizable and one unpolarizable, the former formed by mercury regularly dropping from capillary tube. POLARIZED ELECTRODE : Dropping Mercury Electrode (DME) DEPOLARIZED ELECTRODE : Saturated Calomel Electrode The main principle in the polarography is the reduction process taking place at the electrode. This method has limited sensitivity. The reduction at the electrode increases the voltage applied between the polarisable and non-polarisable electrodes and the current is recorded that is, the metallic ions are reduced at the surface of the electrode. Then the following three steps are observed: Migration of the ions from the solution to the electrode surface. Reduction of ions to form neutral atoms. Deposited atoms are converted to the crystal lattice.

10. PRIN C IPLE Mercury continuously drops from reservoir through a capillary tube into the solution. The optimum interval between drops for most analyses is between 2 and 5 seconds.

11. 11 II. Excitation process A. What happens when a voltage is applied to an electrode in solution containing a redox species? generic redox species O O + e - --> R E = -0.500 V v. SCE Imagine that we have a Pt electrode in sol’n at an initial potential of 0.000 V v. SCE and we switch potential to -0.700 V. First: solvent O = redox supporting electrolyte Pt   O O O      E app =0.0  O O

12. 12 B. Events that happen 1. supporting electrolyte forms an electrical double layer cation movement to electrode causes an initial spike in current Formation of double layer is good because it ensures that no electric field exists across whole sol’n (requires 100:1 conc ratio of supporting elyte:redox species). Pt   O O O      E app = -0.7     O O double layer acts as a capacitor

13. 13 2. Electron transfer reaction How does more O get to electrode surface? mass transport mechanisms O is converted to R at electrode surface. Pt   O O O      E app = -0.7     O O O  R  R A depletion region of O develops - a region in which conc of O is zero. {

14. 14 C. Mass transport to the electrode 1 . Migration - movement in response to electric field. We add supporting electrolyte to make analyte’s migration nearly zero. (fraction of current carried by analyte  zero) 2. Convection stirring 3. Diffusion In experiments relying upon diffusion

15. 15 Solutions and electrodes 1. Solutions: redox couple + solvent + supporting electrolyte supporting elyte: salt that migrates and carries current, and doesn’t do redox in your potential window of interest a wide potential window is desirable water - good for oxidations, not reductions except on Hg supporting elytes: lots of salts nonaqueous solvents: acetonitrile, dimethylformamide, etc. supporting electrolytes: tetraalkylammonium BF 4 , PF 6 , ClO 4 Oxygen is fairly easily reduced - we remove it by deoxygenating with an inert gas (N 2 , Ar).

16. Why MERCURY?  Mercury as working electrode is useful because: It displays a wide negative potential range Its surface is readily regenerated by producing a new drop or film Many metal ions can be reversibly reduced into it.

17. 17 Supporting Electrolyte Polaragrams are recorded in the presence of a relatively high concentration of a base electrolyte such as KCI. The base electrolyte will decrease the resistance for the movement of the metal ions to be determined thus, the IR drop throughout the cell will be negligible. It helps also the movement of ions towards the electrode surface by diffusion only. The discharge potential of the base electrolyte takes place at a very low negative potential therefore, most ions will be reduced before the base electrolyte species.

18. POLAROGRAPHIC DATA  Obtained from an automatic recording instrument is called a polarogram , and the trace is called a polarographic wave.  POLAROGRAM is a graph of current versus potential in a polarographic analysis. 3 categories: collectively referred to as residual current (impurities and supporting electrolyte) referred to as diffusion current resulting from the reduction of the sample called the limiting current The diffusion current of a known concentration of reference standard are first determined followed by the determination of the diffusion current of the unknown concentration.

19. POLAROGRAM  i r (residual current) which is the current obtained when no electrochemical change takes place.  i av (average current/limiting current)is the current obtained by averaging current values throughout the life time of the drop while  i d (diffusion current) which is the current resulting from the diffusion of electroactive species to the drop surface.

20. Factors affecting limiting current Residual current It is the sum of the relativity larger condenser current (charging current) and a very small faradic current. MIGRATION CURRENT It is due to migration of cations from the bulk of the solution towards cathode due to diffusive force . Irrespective of concentration gradient Kinetic current It is proportional to rate constant and volume of interface, hence direct function of size of mercury drop but independent of velocity of flow of mercury from capillary Diffusion current is due to the actual diffusion of electroreducible ion from the bulk of the sample to the surface of the mercury droplets due to concentration gradient

21. Diffusion current – ilkovic equation The value of diffusion current is given by i d = 607.n.D 1/2 .C. M 2/3 .t 1/6    D  is the diffusion coefficient of the ions in the medium (cm 2 /s),  n  is the number of electrons exchanged in the electrode reaction,  m  is the mass flow rate of Hg through the capillary (mg/sec), t  is the drop lifetime in seconds, c  is depolarizer concentration in mol/cm 3 . Curvature of electrode is not considered hence modified by Lingane and Loveridge i d = 607.n.D 1/2 .C. M 2/3 .t 1/6 (1+39D 1/2 m- 1/2 t 1/6 ) Difference between linear and spherical diffusion.

22. Factors affecting diffusion current CONCENTRATION : Diffusion current is directly proportional to concentration of the electroreducible ions . This forms the basis quantitative analysis. i.e , if concentration is less , then diffusion current is less . If concentration is more then diffusion current also more Diffusion of ions is being affected by temperature hence diffusion current also varies with respect is temperature (directly proportional) Viscosity of the medium- inversely proportional Dimensions of capillary Molecular or ionic state of elecro -active species Pressure on the dropping mercury elecrode Temperature

23. Half wave potential Qualitative and quantitative analysis Potential recorded at mid-point of the diffusion current wave Ox+ne - red E=E + (0.0591/n) log (ox)/(red)

24. Factors affecting half wave potential For the half wave potential (HWP), temp coefficient is mostly between +2 and -2 mV/degree HWP of reversible wave not depend on m and t HWP of irreversible wave depend on t for cathodic wave it becomes more + as t increases Changes in the conc and nature of supporting elecctrolyte directly affect HWP pH of the supporting electrolyte imp for oxidation and reduction reaction Complex formation. Rate of electron transfer. Salt concentration.

25. instrumentatiion The apparatus consists of a dropping mercury electrode which acts as a cathode and as a working electrode. The anode used is the pool of mercury at the bottom of the reservoir which acts as a reference electrode. The reference electrode potential is constant. These two electrodes are placed in the sample solution which contains the both anions and cations . Then these anode and cathode are connected to the battery, voltammeter and galvanometer. Then apply the constant voltage and record the current–voltage curves using recorders.

26. The sample cell is made of glass with tapering edge to place the mercury. The cathode capillary is dipped into the sample solution by setting the drop time of about 2–7 s. To control the movement of the ions to the surface on the electrode, the supporting electrolytes such as saturated potassium chloride solution are used. The oxygen present in the sample solution is removed by the alkaline pyrogallol solution. The determined diffusion current is directly proportional to the concentration of the sample solution. The current–voltage curves have the following advantages: Surface area is calculated by the weight of the drops. Reproducible values. Reduction potential is less.

27. Electrodes Electrodes: The polarography is mainly composed of the three types of the electrodes. They are as follows: Working electrodes : The working electrode is mainly used for the determination of the analyte response to the potential. Example: Dropping mercury electrode Dropping mercury electrode: This electrode was first introduced by the Barker. The basic principle involved in this electrode is to control mercury flow through the capillary tube which is closed by the needle valve. Advantages: this electrode is applicable to +0.4 to −1.8 V. Surface area is reproducible Constant renewal of electrode surface, poisoning effect can be removed Reduction of alkali metal ions can be obsevered due to large hydrogen overvoltage on Hg Surface area can be calculated by weght of drop Steady value is obtained by diffusion current Disadvantage: It can be oxidised easily hence avoided to used as anode capillary blocking

28. Auxiliary electrode: It completes the circuit between the potentiostat and the working electrode. Examples: Platinum electrode, Glass carbon electrode Reference electrode: internal and external – External – kept separated from solution through salt bridge or porous membrane Internal: directly in contact with solution, preferred when high negative potential is required or salt bridge material affect adversly It is made by coiling of around 15-20 cm of gauze silver wire into helix, coiling around DME This electrode provides the reference potential for the working electrode and for the auxiliary electrode. Examples: Silver–silver chloride electrode, Calomel electrode Silver electrode is not effective in solution containing cyanide, thiosulphate , ammmonia and hi concentration of halids , solutions containing only acetates, percholate and nitrate.

29. Effect of oxygen Oxygen dissolved in electrolyte solution easily reduced at dropping mercury electrode Results in producing two waves of approximate equal height and extending over a voltage range

30. Types of polarography Linear scan polaropghraphy Rapid DC polaroghraphy Sampled (or test) DC polaroghraphy Pulse P Normal pulse Differential pulse Square wave polaroghraphy

31. Linear scan or linear sweep Rapid scan Voltammetry is the simplest technique. At the working electrode is applied a rapid potential scanning that varies linearly (20 – 100 mV/s). The scanning starts before the discharging potential and stops afterwards Capacitive current increases when the velocity of scanning is increased and cannot be electronically compensated. Thus the performance of this technique are strongly restricted. Detection limits range at mg/l levels.

32. Rapid DC Polarography Rapid DC Polarography The mercury drops fall down, rhythmically, from the capillary with a imposed rhythm, while a linear scanning is imposed to the electrode. The obtained polarogram is a wave characterised by strong oscillations due to the rhythmic falling of the drop (that means a rhythmic interruption of the electrical circuit).

33. Staircase Voltammetry A different variant of the LSV technique consist in a regular potential step scanning. The current is sampled just before the subsequent step. Thus the signal is less influenced by the capacitive current.

34. 37 Pulse Polarography Normal Pulse polarog . : gradual increase in the amplitude in the voltage pulse Differential pulse polarog .: Voltage pulse of constant amplitude superimposed on a slowly increasing voltage squarewave voltammetry : which can be considered a special type of differential pulse voltammetry in which equal time is spent at the potential of the ramped baseline and potential of the superimposed pulse.

35. 38                                                                              Series of pulses (40 ms duration) of increasing amplitude (potential) are applied to successive drops at a preselected time (60 ms) near the end of each drop lifetime. Between the pulses, the electrode is kept at a constant base potential where no reaction occurs i c is very large at the beginning of the pulse; it then decays exponentially. i is measured during the 20 ms of the second half of the pulse when i c is quite small The current is sampled once during each drop life and stored until next sample period, thus the polarogram shows a staircase appearance NPP is designed to block electrolysis prior to the measurement period Normal Pulse polarography

36. 39 Differential Pulse Polarography A pulse (of constant amplitude of 5-100 mV) of 40-60 ms is applied during the last quarter of the drop life The pulse is superimposed on a slowly increasing linear voltage ramp. The current is measured twice: one immediately preceding the pulse and the other near the end of the pulse. Overall response plotted is the difference between the two currents sampled

37. 40                                                                              Fixed magnitude pulses (50 mV each) superimposed on a linear potential ramp are applied to the working electrode at a time just before the drop falls (last 50 ms). The current is measured at 16.7 ms prior to the DC pulse and 16.7 ms before the end of the pulse.

38. 41                                                                             

39. 42 Voltamunogram for a differential pulse polarography experiment

40. AD V AN T AGES Simple sample handling Speed of analysis High sensitivity Comparable or better accuracy Cheaper instrumentation and lower cost of chemicals used Limited used of environmentally unfriendly organic solvents

41. disadvantages Less accurate. Skilled person is required.

42. applications APPLICATIONS Used in the determination of the composition of the alloys. Used in the qualitative determination of the elements. Used in the estimation of the trace metals like Zn, Fe, Mn and Cu. Used in the determination of the free sulfur in petroleum fractions. Used in the determination of the vitamin C in the food beverages. Used in the functional group analysis. Used in the determination of the complex compositions. Used in the determination of the dissolved oxygen in the gases. Used in the determination of the local anesthetics ( dyclonine ).